Age determination technique and tools

Most surveys ask for the age and date of birth of each participant. For participants up to 59 months of age, the date of birth and date of visit should be recorded.1 For individuals 6 years of age and above, age can be estimated in completed years, that is, the age at the time of the person’s last birthday. If the respondent (ideally the main caregiver of the child) knows a child’s age in months or years this should be entered in the appropriate section on the form. If data are being collected electronically, the electronic device should be programmed to calculate the number of years and months from the reported date of birth to the date of the interview. If paper-based forms are used, the interviewer should record the date of birth and the date of the visit on the questionnaire for later use in a tool developed for determining age in months or years. These calculated ages should be cross-checked with the age provided by the caregiver.

The age of a child is crucial for making and interpreting appropriate anthropometric measurements. This underscores the importance of probing for an accurate date of birth. Even where the date of birth is included on a child health card or similar document, it is possible that it was incorrectly recorded or that the writing may make it difficult to determine, therefore the date of birth should always be confirmed with the caregiver. If the primary respondent does not have the necessary information, the interviewer should ask other household members.

These are the steps for estimating the age of a child under 6 years, using a recorded birthdate:1

  • Ask the respondent for documentary evidence of the child’s date of birth (such as a birth certificate, child health card or holy book).
  • Record the day, month and year of birth as noted on the documentary evidence, and indicate the type of documentary evidence that was provided. Even if the respondent recalls the child’s date of birth, politely ask to see a copy of the documentary evidence and record the information directly from it.
  • If no document is available, ask the respondent for the date of birth as they recall it and indicate the source on the questionnaire as “respondent’s report”.
  • If the respondent does not know the child’s exact date of birth, then at minimum the month and year of birth should be obtained using a local events calendar. The local events calendar will have been prepared and tested previously, and all anthropometrists should have been trained its use. See the Age determination for children under 6 years of age online tool for more guidance on estimating the month and year of birth using the calendar of events and indexing techniques.

Whether using documentary evidence or the respondent’s report, the anthropometric team should record the actual date of birth, if specified. If using the local events calendar, it is probable that it will be impossible to identify the exact date of birth. In this case, anthropometrists should enter “00 00 00” (unknown) for the date of birth and enter the birth month and year as determined by the local events calendar. The source of the information should always be recorded in the questionnaire.

For children aged 6 to 18 years, follow the same steps as for children under 6 years to get the best estimate of the date of birth. If it is not possible to get the exact date, the month and year of birth are adequate. For adults over 18 years of age, the year of birth is adequate.

  1. Recommendations for data collection, analysis and reporting on anthropometric indicators in children under 5 years old. Geneva: World Health Organization and the United Nations Children’s Fund (UNICEF); 2019. Licence: CC BY-NC-SA 3.0 IGO,, accessed 15 July 2019).  2